1. Field of the Invention
This invention relates to a method for the continuous hydrolysis of organochlorosilane and more particularly, to a method for the continuous hydrolysis of organochlorosilane comprising the steps of feeding organochlorosilane from a first stage of hydrolysis, maintaining constant the level of a hydrogen chloride-containing aqueous medium in each stage, and feeding an amount of water corresponding to the consumption by reaction from the last stage.
2. Prior Art
Various silicone products as typified by silicon oil and silicone rubber are produced from diorgano-hydrolyzate intermediates which are produced by hydrolysis and polycondensation of diorganodichlorosilane. In this process, since the quality of silicone products is directly governed by the hydrolysis reaction, those skilled in the art must pay attention to the physical properties and state of hydrolyzate and the stability of operation. For example, if the proportion of linear polysiloxane having a high molecular weight increases, the loads for phase separation and transfer increase. Therefore, for carrying out stable operation, it is desirable to produce cyclic polysiloxane in relatively high yields.
The above reaction generates hydrogen chloride as a by-product. Since hydrogen chloride is a valuable raw material for the synthesis of methyl chloride, it is also important to increase the recovery rate of hydrogen chloride.
Paying attention to these problems, those skilled in the art have sought for industrially more advantageous operation.
For example, U.S. Pat. No. 2,758,124 discloses a method of continuously feeding organochlorosilane and hydrochloric acid into a reactor, thereby producing linear and cyclic polysiloxanes and an aqueous hydrogen chloride solution having a concentration of 25 to 36% by weight. The aqueous hydrogen chloride solution thus generated is distilled and separated into a hydrogen chloride-water azeotrope having a hydrogen chloride concentration of 21% by weight and anhydrous hydrogen chloride for reuse. According to this method, linear or cyclic polysiloxane having a hydroxyl group at an end is obtained as a product, the amounts of terminal Cl.sup.- groups and residual hydrogen chloride concomitant with the polysiloxane product can be reduced. This method, however, is disadvantageous from the energy standpoint because distillation operation is carried out upon recovery of hydrogen chloride. Since a noticeable amount of organopolysiloxane is dissolved in the recovered aqueous hydrogen chloride solution, the subsequent steps must include washing steps, complicating the process. Continuous operation is sometimes interrupted due to scale deposition.
To avoid these drawbacks, U.S. Pat. No. 4,382,145 discloses a method of hydrolyzing organochlorosilane, utilizing an aqueous hydrochloric acid solution containing at least 35% by weight of hydrogen chloride, as the source of water for the organochlorosilane hydrolysis, in an amount sufficient to provide a molar ratio of water to organochlorosilane of from about 10 to about 30. This method yields a polysiloxane mixture, anhydrous hydrogen chloride, and an aqueous medium containing a saturated amount of hydrogen chloride, saving the majority of energy required for the recovery of hydrogen chloride. However, if the aqueous medium containing a saturated amount of hydrogen chloride is used, terminal Cl.sup.- groups and residual hydrogen chloride are left in the organopolysiloxane mixture, leading to a substantial drop of reactivity.
The drop of reactivity not only introduces inconvenience into subsequent steps, but causes a substantial drop in phase separation between the polysiloxane product and the aqueous medium containing hydrogen chloride, increasing the loads for phase separation and transfer. Further, since terminal Cl.sup.- groups or hydrogen chloride left in the organopolysiloxane is washed away in the subsequent step, a substantial amount of hydrogen chloride is lost together with the wash liquid.
U.S. Pat. No. 4,609,751 discloses a method of hydrolyzing organochlorosilane in substantially a stoichiometric equivalence of water. This method is advantageous from the energy standpoint since anhydrous hydrogen chloride can be readily obtained. This method, however, is disadvantageous in phase separation and effective utilization of hydrogen chloride as in the previous methods.
U.S. Pat. No. 5,476,916 discloses a method of carrying out the hydrolysis of alkylchlorosilane in two stages. In the first stage, alkylchlorosilane is reacted with water in aqueous hydrochloric acid to produce anhydrous hydrogen chloride and polysiloxanes. In the second stage, water is supplied as steam to reduce the chlorine content of the polysiloxanes. Since a large amount of water must be fed as steam into the reaction system, an excessive quantity of energy is necessary and the system becomes complex.
Although those skilled in the art have made investigations on the continuous hydrolysis of organochlorosilanes, a method capable of energy saving and stable operation has yet never been available.
The hydrogen chloride-containing aqueous medium resulting from the hydrolysis step of diorganodichlorosilane contains siloxane which is present partially dissolved and partially dispersed in the aqueous medium and in a concentration ranging from several tens of ppm to about 10,000 ppm depending to the hydrogen chloride concentration. This makes more difficult the reuse of the hydrogen chloride-containing aqueous medium and causes to incur a loss of siloxane. In the event where the aqueous medium is drained after waste water treatment including a neutralizing step with alkali solution, a large amount of alkali is necessary, prevention of siloxane run-off is very difficult, and the environmental influence is serious.
Therefore, it is an industrially important task to carry out hydrolysis of organochlorosilane in an energy-saving manner while maintaining the physical properties of hydrolyzate and stable operation and to recover substantially the entire amount of hydrogen chloride generated in the reaction system as anhydrous hydrogen chloride.